the ongoing health scare for hillary clinton. She remains in a new york hospital tonight, her daughter chelsea by her side today. And now her doctors have revealed it is a blood clot near the brain that's keeping her in the hospital. Tonight, we tackle many questions here, how was it discovered, how will they treat it, what is her prognosis. Dr. Besser is standing by, but first, abc's martha raddatz in washington. And martha, this is a lot worse than many thought. Reporter: It sure seems to be, david. We waited 24 hours for detail about this clot, but now, for the first time, we know the clot is not inclinton's leg, it is in a vein next to her brain. Tonight, the normally globetrotting secretary of state is in a hospital room, monitored closely and being given blood thinners to break up the clot. Family members including daughter chelsea have been at her side. The clot was discovered on an mrio sunday. A statement from her doctors tonight, saying, "the scan revealed a clot in the vein that is situated in the space between the brain and the skull behind the right ear." The statement says the secretary did not suffer a stroke or any neurological damage. But this is clearly a serious setback. It was december 7th when the secretary was last seen in public, during an overseas tour where she picked up a bad stomach vi days later, still ill, the secretary fainted and got that nasty concussion. Clinton has had a blood clot in the past. In 1998, b her right knee, after a year of heavy travel. This clot comes at the end of the most demanding period of her life, as she recently told abc's barbara walters. Are you exhausted? I am. To be honest, I am, because when I do something, I really want to do it. I want to do it to the best of my ability. That means I pretty much work all the time. Reporter: But what is unclear tonight is whether she will be able to return to her full time role at the state department before a new secretary of state is sworn in. Clinton's doctors say she will be released from the hospital once the proper medication dose is determined. They do say she is making excellent progress and they are confident she will eventually make a full recovery. She is reportedly in good spirits, david, engaging with her doctors, her family and her staff. That is encouraging to hear. We know chelsea was there earlier today. Martha, thank you. I want to bring in dr. Richard besser, also covering this. You said today, make no mistake, this is very serious. What exactly is it she's facing? This is much more serious than what we've been talking about. Take a look at this picture. She had a blood clot in a very important vein that runs along her brain, right behind her ear. Now, if they hadn't caught this, the clot in there could have gotten larger and that ultimately could have been fatal. Because it's a vein next to the brain and could have applied pressure to the brain had it gotten much larger. You mentioned the mri. We heard it from martha there. What led to the mri? That's a very good question. Normally you don't get a routine mri follo concussion. So, I expect that her symptoms led them to put her in the mri and detect this. And you told me about the blood thinners they are using to treat her. But this is very tricky, too. Right. The goal is to use the blood thinners and hopefully the clot will resolve. But it's a balance between keeping her blood thin but making sure she doesn't have bleeding into her brain or into other parts of her body that could be very severe.

This transcript has been automatically generated and may not be 100% accurate.

Now Playing: Dramatic Break From South Carolina Maximum Security Prison

Now Playing: Last Stand For Rebels Holding Out in Aleppo

Now Playing: Trump Takes Aim at Carrier Union Boss

Now Playing: John Glenn in His Own Words

Now Playing: Tracking a Deadly Snowstorm and an Arctic Blast from the Rockies to the East

Now Playing: Heated Debate Over Shooting of High School Teen at Reno School

Now Playing: Holiday Price-Fixing?

Now Playing: John Glenn: Space Pioneer, Trailblazer and American Hero Dead at 95

Now Playing: Manhunt Underway for Suspected Cop Killer in Georgia

Now Playing: Index: Bomb Squad Called to Investigate Object on Pasadena Freeway